Electrical components are generally secured to a circuit board or other substrate by means of a soldering operation. Although there are a number of common soldering processes to secure components to the substrate, a conventional soldering process may be comprised of three separate steps. These steps include (1) applying flux to the substrate, (2) preheating the substrate, and (3) soldering various components to the substrate. In some situations, such as reflow and surface mounting processes, preheating is unnecessary. As some examples, the invention pertains to component securement in applications utilizing circuit boards, micropalates, interposer boards, controlled collapse chip collections, VGA and other computer chips.
Soldering flux is a chemical compound which promotes the wetting of a metal surface by molten solder. The flux removes oxides or other surface films from the base metal surface. The flux also protects the surface from reoxidation during soldering and alters the surface tension of the molten solder and the base material. Substrates, such as printed circuit boards, must be cleaned with flux to effectively prepare the board for soldering and to properly wet the electrical components to be secured to the circuit board.
During the soldering operation it may be necessary to dispense minute amounts or droplets of solder flux onto discrete portions of the substrate. Various types of dispensers have been used for this purpose, such as syringe style contact dispensers and valve-operated, noncontact dispensers. In addition to solder flux, other liquids may also be applied to the substrate. These liquids may include adhesives, solder paste, solder mask, grease, oil encapsulants, potting compounds, inks and silicones.
Because of surface tension effects, liquid exiting a valve-operated, noncontact dispenser typically forms a substantially spherically-shaped, airborne droplet before reaching the substrate. The droplet therefore contacts the substrate in a specific, generally circular surface area. Depending upon the viscosity and surface tension characteristics of the droplet material, the droplet may maintain a substantially semi-spherical shape above the surface contact area. For instance, if the droplet material has a high viscosity or high surface tension, the droplet will generally maintain a semi-spherical shape above the surface of the substrate and the surface contact area will be relatively small. For conventional fluxes, the height of the droplet may generally equal the diameter of the droplet. If, however, the droplet material has relatively low viscosity or low surface tension, the spherical shape flattens out onto the surface and the surface contact area is greater. In essence, high viscosity droplets or those with high surface tension do not spread out over the surface like low viscosity droplets or those with low surface tension.
During the manufacture of electronic devices, it is desirable to use the smallest effective amount of flux possible while still covering the greatest amount of surface area with the flux. In many soldering operations, the flux is best applied to a substrate in the form of a series of droplets on discrete areas of the substrate. It is preferable that the single droplet of flux flatten out and form a thin layer over a larger area of the substrate. A relatively thin layer of solder flux has several advantages relative to a thicker layer of flux. For example, a thin layer of solder flux yields more reliable solder connections between the electrical components and, for example, a printed circuit on the substrate, especially where "no clean" fluxes are used. A thin layer formed from a single droplet of flux also uses less flux than several taller droplets of flux used to cover the same area. Also, a single droplet of flux that spreads out to form a thin layer increases manufacturing throughput because applying a single flattened droplet is quicker than covering the same surface area with several taller droplets.
Since solder flux generally has high surface tension, it does not flatten appreciably upon contact with the substrate. Instead, the noncontact dispensing operation leaves a relatively tall droplet with a substantially semi-spherical shape and a small contact area. As a result, it is difficult to produce a thin layer of solder flux using conventional noncontact dispensers and conventional solder flux.
Therefore, it would be desirable to provide a noncontact droplet dispenser which is able to both dispense a droplet of viscous liquid, such as solder flux, and flatten or spread out the droplet onto a substrate to increase its surface contact area.